One problem encountered when handling small fluid volumes is to effectively mix different fluids together. Poor mixing can occur in channel-based microfluidic devices, where two or more volumes of different fluids, each flowing through microchannels, are combined together at a junction and into a single channel. In some cases, poor mixing can be ameliorated by introducing flow diverters into the junction or the single channel to redirect the flow of the two fluids to facilitate better mixing. However, flow diverters are complex structures that are technically difficult to construct. Additionally, channels having flow diverters are prone to being clogged by particles suspended in the fluid.
Poor mixing can also occur in droplet-based microfluidic devices, where the fluids are not confined in channels. Rather, small droplets of fluid (e.g., fluid volumes of about 100 microliters or less) are moved and mixed together on a surface. In some cases, it is desirable to add a small volume of a reagent to a sample droplet to facilitate the analysis of the sample, without substantially diluting it. In such cases, there is limited ability to mix two droplets together because there is no flow of fluids to facilitate mixing. Additionally, because there is no flow of fluids, it is not possible to facilitate mixing in droplets with the use of flow diverters.
Embodiments of the present invention overcome these problems by providing a device that facilitates the movement and mixing of small volumes of fluids, as well as providing methods of using and making such a device.